We propose to use the resources of two large well-characterized cohort studies (the Nurses' Health Study I and II) to assess genotypic risk factors for breast cancer. We will quantify the association of rare HRAS1 alleles with breast cancer and test whether this association is modified by known breast cancer risk factors. We will assess the frequency of mutations in BRCA1 among cases from the cohort, and test whether the distribution of reproductive and other breast cancer risk factors is different between cases with and without BRCA1 germline mutations. These analyses will be nested in the subcohort of 33,000 women from Nurses' Health Study I who gave blood samples in 1989-90, and thus, will be among the few studies able to prospectively examine these issues in a defined cohort with complete ascertainment of incident cases and comprehensive prospective information on other breast cancer risk factors. In addition, we will obtain blood specimens from cases and controls in Nurses' Health Study II, a cohort of younger women. Across both studies we expect to genotype 1,280 cases. Matching two controls to each case we will have 98% power to detect a relative risk of 2.0, and 80% power to detect a relative risk of 1.5 associated with the rare HRAS1 alleles genotype. We will have substantial power to examine interactions of HRAS1 alleles with other breast cancer risk factors. Although the cost of the BRCA1 assays and rarity of the mutations precludes assaying controls for BRCA1 mutations, by determining the spectrum of BRCA1 mutations in a defined case series, we will contribute information on the pathogenesis of individual mutations. We will have adequate power in case-case analyses to detect moderate interactions between BRCA1 mutations and other breast cancer risk factors. The results from these studies will provide firm information on these genetic markers of risk, as well as defining reproductive and lifestyle factors which interact with them. The availability of this information is critical to our ability to appropriately counsel women with these genotypes.